Computer Network Performance Issues




PERFORMANCE ISSUES

Performance issues are very important in computer networks. When hundreds or thousands of computers are interconnected, complex interactions, with unforeseen consequences, are common. Frequently, this complexity leads to poor performance and no one knows why. In the following sections, we will examine many issues related to network performance to see what kinds of problems exist and what can be done about them.

Unfortunately, understanding network performance is more an art than a science. There is little underlying theory that is actually of any use in practice. The best we can do is give some rules of thumb gained from hard experience and present examples taken from the real world. We have delayed this discussion until we studied the transport layer because the performance that applications receive depends on the combined performance of the transport, network and link layers, and to be able to use TCP as an example in various places.

In the next sections, we will look at six aspects of network performance:

1. Performance problems.

2. Measuring network performance.

3. Host design for fast networks.

4. Fast segment processing.

5. Header compression.

6. Protocols for ‘‘long fat’’ networks.

These aspects consider network performance both at the host and across the network, and as networks are increased in speed and size.

Performance Problems in Computer Networks

Some performance problems, such as congestion, are caused by temporary resource overloads. If more traffic suddenly arrives at a router than the router can handle, congestion will build up and performance will suffer. We studied congestion in detail in this and the previous chapter.

Performance also degrades when there is a structural resource imbalance. For example, if a gigabit communication line is attached to a low-end PC, the poor host will not be able to process the incoming packets fast enough and some will be lost. These packets will eventually be retransmitted, adding delay, wasting bandwidth, and generally reducing performance.

Overloads can also be synchronously triggered. As an example, if a segment contains a bad parameter (e.g., the port for which it is destined), in many cases the receiver will thoughtfully send back an error notification. Now consider what could happen if a bad segment is broadcast to 1000 machines: each one might send back an error message. The resulting broadcast storm could cripple the network. UDP suffered from this problem until the ICMP protocol was changed to cause hosts to refrain from responding to errors in UDP segments sent to broadcast addresses. Wireless networks must be particularly careful to avoid unchecked broadcast responses because broadcast occurs naturally and the wireless bandwidth is limited.

A second example of synchronous overload is what happens after an electrical power failure. When the power comes back on, all the machines simultaneously start rebooting. A typical reboot sequence might require first going to some (DHCP) server to learn one’s true identity, and then to some file server to get a copy of the operating system. If hundreds of machines in a data center all do this at once, the server will probably collapse under the load.

Even in the absence of synchronous overloads and the presence of sufficient resources, poor performance can occur due to lack of system tuning. For example, if a machine has plenty of CPU power and memory but not enough of the memory has been allocated for buffer space, flow control will slow down segment reception and limit performance. This was a problem for many TCP connections as the Internet became faster but the default size of the flow control window stayed fixed at 64 KB.

Another tuning issue is setting timeouts. When a segment is sent, a timer is set to guard against loss of the segment. If the timeout is set too short, unnecessary retransmissions will occur, clogging the wires. If the timeout is set too long, unnecessary delays will occur after a segment is lost. Other tunable parameters include how long to wait for data on which to piggyback before sending a separate acknowledgement, and how many retransmissions to make before giving up.

Another performance problem that occurs with real-time applications like audio and video is jitter. Having enough bandwidth on average is not sufficient for good performance. Short transmission delays are also required. Consistently achieving short delays demands careful engineering of the load on the network, quality-of-service support at the link and network layers, or both.

Network Performance Measurement

When a network performs poorly, its users often complain to the folks running it, demanding improvements. To improve the performance, the operators must first determine exactly what is going on. To find out what is really happening, the operators must make measurements. In this section, we will look at network performance measurements. Much of the discussion below is based on the seminal work of Mogul (1993).

Measurements can be made in different ways and at many locations (both in the protocol stack and physically). The most basic kind of measurement is to start a timer when beginning some activity and see how long that activity takes. For example, knowing how long it takes for a segment to be acknowledged is a key measurement. Other measurements are made with counters that record how often some event has happened (e.g., number of lost segments). Finally, one is often interested in knowing the amount of something, such as the number of bytes processed in a certain time interval.

Measuring network performance and parameters has many potential pitfalls. We list a few of them here. Any systematic attempt to measure network performance should be careful to avoid these.

Make Sure That the Sample Size Is Large Enough

Do not measure the time to send one segment, but repeat the measurement, say, one million times and take the average. Startup effects, such as the 802.16 NIC or cable modem getting a bandwidth reservation after an idle period, can slow the first segment, and queueing introduces variability. Having a large sample will reduce the uncertainty in the measured mean and standard deviation. This uncertainty can be computed using standard statistical formulas.

Make Sure That the Samples Are Representative

Ideally, the whole sequence of one million measurements should be repeated at different times of the day and the week to see the effect of different network conditions on the measured quantity. Measurements of congestion, for example, are of little use if they are made at a moment when there is no congestion. Sometimes the results may be counterintuitive at first, such as heavy congestion at 11 A.M., and 1 P.M., but no congestion at noon (when all the users are at lunch).

With wireless networks, location is an important variable because of signal propagation. Even a measurement node placed close to a wireless client may not observe the same packets as the client due to differences in the antennas. It is best to take measurements from the wireless client under study to see what it sees. Failing that, it is possible to use techniques to combine the wireless measurements taken at different vantage points to gain a more complete picture of what is going on (Mahajan et al., 2006).

Caching Can Wreak Havoc with Measurements

Repeating a measurement many times will return an unexpectedly fast answer if the protocols use caching mechanisms. For instance, fetching a Web page or looking up a DNS name (to find the IP address) may involve a network exchange the first time, and then return the answer from a local cache without sending any packets over the network. The results from such a measurement are essentially worthless (unless you want to measure cache performance).

Buffering can have a similar effect. TCP/IP performance tests have been known to report that UDP can achieve a performance substantially higher than the network allows. How does this occur? A call to UDP normally returns control as soon as the message has been accepted by the kernel and added to the transmission queue. If there is sufficient buffer space, timing 1000 UDP calls does not mean that all the data have been sent. Most of them may still be in the kernel, but the performance test program thinks they have all been transmitted.

Caution is advised to be absolutely sure that you understand how data can be cached and buffered as part of a network operation.

Be Sure That Nothing Unexpected Is Going On during Your Tests

Making measurements at the same time that some user has decided to run a video conference over your network will often give different results than if there is no video conference. It is best to run tests on an idle network and create the entire workload yourself. Even this approach has pitfalls, though. While you might think nobody will be using the network at 3 A.M., that might be when the automatic backup program begins copying all the disks to tape. Or, there might be heavy traffic for your wonderful Web pages from distant time zones. 

Wireless networks are challenging in this respect because it is often not possible to separate them from all sources of interference. Even if there are no other wireless networks sending traffic nearby, someone may microwave popcorn and inadvertently cause interference that degrades 802.11 performance. For these reasons, it is a good practice to monitor the overall network activity so that you can at least realize when something unexpected does happen.

Be Careful When Using a Coarse-Grained Clock

Computer clocks function by incrementing some counter at regular intervals. For example, a millisecond timer adds 1 to a counter every 1 msec. Using such a timer to measure an event that takes less than 1 msec is possible but requires some care. Some computers have more accurate clocks, of course, but there are always shorter events to measure too. Note that clocks are not always as accurate as the precision with which the time is returned when they are read.

To measure the time to make a TCP connection, for example, the clock (say, in milliseconds) should be read out when the transport layer code is entered and again when it is exited. If the true connection setup time is 300 μsec, the difference between the two readings will be either 0 or 1, both wrong. However, if the measurement is repeated one million times and the total of all measurements is added up and divided by one million, the mean time will be accurate to better than 1 μsec.

Be Careful about Extrapolating the Results

Suppose that you make measurements with simulated network loads running from 0 (idle) to 0.4 (40% of capacity). For example, the response time to send a voice-over-IP packet over an 802.11 network might be as shown by the data points and solid line through them in Fig. 6-49. It may be tempting to extrapolate linearly, as shown by the dotted line. However, many queueing results involve a factor of 1/(1 − ρ), where ρ is the load, so the true values may look more like the dashed line, which rises much faster than linearly when the load gets high. That is, beware contention effects that become much more pronounced at high load.



Frequently Asked Questions

+
Ans: We have saved one of the key functions of TCP for last: congestion control. When the load offered to any network is more than it can handle, congestion builds up. The Internet is no exception. The network layer detects congestion when queues grow large at routers and tries to manage it, if only by dropping packets. view more..
+
Ans: As mentioned earlier, window management in TCP decouples the issues of acknowledgement of the correct receipt of segments and receiver buffer allocation. For example, suppose the receiver has a 4096-byte buffer, as shown in Fig. 6-40. view more..
+
Ans: Figure 6-36 shows the layout of a TCP segment. Every segment begins with a fixed-format, 20-byte header. The fixed header may be followed by header options. view more..
+
Ans: Performance issues are very important in computer networks. When hundreds or thousands of computers are interconnected, complex interactions, with unforeseen consequences, are common. view more..
+
Ans: Measuring and tinkering can improve performance considerably, but they cannot substitute for good design in the first place. A poorly designed network can be improved only so much. view more..
+
Ans: We have been looking at fast networks for too long. There is more out there. Let us now consider performance on wireless and other networks in which bandwidth is limited. view more..
+
Ans: We will finish this chapter by describing a new kind of transport that may one day be an important component of the Internet. TCP and most other transport protocols are based on the assumption that the sender and the receiver are continuously connected by some working path, or else the protocol fails and data cannot be delivered. view more..
+
Ans: Having finished all the preliminaries, we now come to the layer where all the applications are found. The layers below the application layer are there to provide transport services, but they do not do real work for users. In this chapter, we will study some real network applications. view more..
+
Ans: In theory at least, a single name server could contain the entire DNS database and respond to all queries about it. In practice, this server would be so overloaded as to be useless. Furthermore, if it ever went down, the entire Internet would be crippled. view more..
+
Ans: Electronic mail, or more commonly email, has been around for over three decades. Faster and cheaper than paper mail, email has been a popular application since the early days of the Internet. Before 1990, it was mostly used in academia. view more..
+
Ans: Now we turn from the user interface to the format of the email messages themselves. Messages sent by the user agent must be placed in a standard format to be handled by the message transfer agents. view more..
+
Ans: Now that we have described user agents and mail messages, we are ready to look at how the message transfer agents relay messages from the originator to the recipient. The mail transfer is done with the SMTP protocol. view more..
+
Ans: Our mail message is almost delivered. It has arrived at Bob’s mailbox. All that remains is to transfer a copy of the message to Bob’s user agent for display. This is step 3 in the architecture of Fig. 7-7. This task was straightforward in the early Internet, when the user agent and mail transfer agent ran on the same machine as different processes. view more..
+
Ans: The Web, as the World Wide Web is popularly known, is an architectural framework for accessing linked content spread out over millions of machines all over the Internet. view more..
+
Ans: The basis of the Web is transferring Web pages from server to client. In the simplest form, Web pages are static. That is, they are just files sitting on some server that present themselves in the same way each time they are fetched and viewed. view more..
+
Ans: The static page model we have used so far treats pages as multimedia documents that are conveniently linked together. It was a fitting model in the early days of the Web, as vast amounts of information were put online. view more..
+
Ans: Now that we have an understanding of Web content and applications, it is time to look at the protocol that is used to transport all this information between Web servers and clients. It is HTTP (HyperText Transfer Protocol), as specified in RFC 2616. view more..
+
Ans: The Web is used from most every type of computer, and that includes mobile phones. Browsing the Web over a wireless network while mobile can be very useful. It also presents technical problems because much Web content was designed for flashy presentations on desktop computers with broadband connectivity. view more..




Rating - NAN/5
475 views

Advertisements